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Suppose it was possible to travel to inside the event horizon of a black hole safely. Also suppose said black hole is large enough that the curvature of the horizon of the surface of the central mass of the black hole is similar to that on Earth or perhaps the Moon, yet we are somehow able to hand-wave away the problem of the crushing gravity. Also, for simplicity, suppose our intrepid traveller has senses similar to those of a human, and no high-tech sensors (as are seen for example in Star Trek) are used.

Under such circumstances, if our intrepid traveller were to look "up" (away from the surface of the central mass) in a way similar to looking up at the sky on Earth or the Moon, what would s/he see? What would the "sky" look like?

Note that I am not asking about actually travelling to beneath the event horizon safely, or what anything would appear like to an observer at a safe distance outside the event horizon.

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  • $\begingroup$ The 'central mass' occupies no volume - it's an infinitely dense singularity, there is no 'surface', also given that we are uncertain of any events beyond the event horizon, this question is likely unanswerable. $\endgroup$ – Scott Downey Feb 27 '15 at 13:54
  • $\begingroup$ @ScottDowney I disagree, it will be speculative but I'm sure you can extend GR to see how light would travel beyond the event horizon. $\endgroup$ – overactor Feb 27 '15 at 14:02
  • $\begingroup$ I suppose it would be extremely bright, considering that Black holes suck in all the light around them (which is why simulated black holes in computer programs distort the space around them). $\endgroup$ – JDSweetBeat Feb 27 '15 at 16:10
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    $\begingroup$ Relevant questions on Physics: physics.stackexchange.com/questions/111924/…, physics.stackexchange.com/questions/106484/… $\endgroup$ – HDE 226868 Feb 27 '15 at 19:19
  • $\begingroup$ @DustinJackson Except they don't, really. The light, matter and other energy that gets bent into the black hole is that which is directed at or nearly at the black hole's position. As we have discussed previously as well, there really is no magic to black holes. You get basically the same distortion effects as around any large celestial body, including our own sun, just mostly amplified. $\endgroup$ – a CVn Feb 27 '15 at 20:37
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This is where the spatial intuitions from outside the horizon fail us.

As you pass the event horizon, it becomes impossible to move away from the central singularity. This means that gradually, the curvature of spacetime become such that what used to be the forward direction is flipped to become the a time-like future direction, since the singularity cannot be avoided. Likewise, the time-like dimension is now perceived to be a spacelike dimension. After a certain point, if you were somehow able to look 'back', you would see a long series of past yous blocking the view. But you can't look back, because that would imply that atoms or photons would move back away from the singularity.

An imaginary cross-section through a black hole would show a kaleidoscope of various time-like positions inside the black hole.

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A couple of guys from the University of Colorado created a video simulating what it would look like to fall into a black hole, including passing the event horizon. They have several other simulations on their website.

enter image description here

You should also check out the renderings made for the movie Interstellar. They made a couple discoveries while getting the physics simulations right.

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It would be hot. very hot

Large volumes of mass are drawn into the black hole and accelerated. Outside the black hole, the lowest energy matter gets spun off with so much ferocity that it generates X-rays. The higher energy portions get flung downward onto your unsuspecting planet.

I am assuming the planet falls in such a way that it isn't centered on the black hole (I can't figure out how to make that happen), but rather is falling towards it like any other. In this case, time dilation is doing its best to cause the energy emitted to appear to have lower energy, but you're still going to have large volumes of matter crushing in on the sky, with blackbody radiation not only making the sky appear an intense shade of violet (intense as "brighter than sticking a nuclear bomb on your eyeball and setting it off), but also baking the planet with a remarkable bath of higher energy particles like xrays and gamma rays.

Assuming you found a way to see past this haze of bright things, I'm assuming there is some interest in what the event horizon looks like. It doesn't look like anything unusual! The event horizon is only a limit for things escaping the black hole, but does nothing to affect things coming in. The only guarantee you would have is that, if someone held up a mirror outside the event horizon, and you looked at it, you would not be able to see yourself, for the photons from your body never get that far before being bent back into the black hole.

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  • $\begingroup$ "you would not be able to see yourself, for the photons from your body never get that far before being bent back into the black hole." Wouldn't that mean the even horizon would look like a funhouse mirror? light bouncing off you but turning back you would see it right? $\endgroup$ – bowlturner Feb 27 '15 at 15:19
  • $\begingroup$ You might be able to get a very tiny funhouse mirror in the opposite direction of the singularity, but generally speaking there's nothing that will bend it directly back to to you, only in an arc towards the horizon. Most of those photons would simply fall away towards the singularity and $\endgroup$ – Cort Ammon Feb 27 '15 at 15:47
  • $\begingroup$ actually that makes a lot more sense $\endgroup$ – bowlturner Feb 27 '15 at 15:50
  • $\begingroup$ Remember photons are massless, it's not that they're being pulled in by the gravity, its that spacetime has an infinitely deep pocket that the photons go into and don't come out. The light doesn't just fall in like vectors in a pincushion, because of gravitational lensing the incoming light is very much affected, there would be significant distortion to witness from the horizon. So, it would actually look quite unusual. $\endgroup$ – Samuel Feb 27 '15 at 17:42
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It would look like the past

At least if you're talking about the sky that would have been visible from outside the horizon, just before crossing.

I believe the interpretation in the video is flawed, since it does not account for the flow of the space-time structure itself inside the black-hole horizon. Frame dragging and the intense gravity means that a photon cannot travel outward. It's not like you're in a black sphere. As @Aaru correctly pointed out, in effect it's as if the space dimension pointing towards the singularity and a time-dimension change roles. I recall reading of a "river model" of inner-horizon space whereby spacetime itself can be imagined as flowing towards the singularity, dragging us and light along no matter what we do. And that's assuming that no dimensions are extinguished or created, which is unclear to me.

Regardless, I strongly feel that the interpretation of the singularity as a pointlike destination is nonsensical, since our own future is not perceivable as a pointlike destination, and we can't look upon our own past any more than a photon in a black hole could travel outwards.

So if the inside of the sphere is your future, how does the outside look when you're being dragged inwards by a river of time, possibly flowing faster than light, especially when from the perspective of the outside, you've already been subjected to infinite time dilation?

Frankly, the question is beyond my ken, and I suspect, possibly beyond the reach of any physics we know.

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We don't know.

Our current ideas of physics don't really work inside the hole itself. In principle, there's nothing in there except the "signularity" of all the mass at the centre. Except that breaks loads of the "laws" of physics too.

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